After "camping out" overnight in the Quest airlock, which purges nitrogen from their bloodstream to prevent decompression sickness, mission specialists Mike Foreman and Robert Satcher moved out into the vacuum of space at 1424 GMT.

After switching his suit to battery power, signifying the start of his first ever space walk, Satcher rode the station's robotic arm carrying a vital communications antenna, which he and Foreman then bolted into place.

Lead spacewalker Foreman, wearing a suit with solid red stripes, and Satcher, in an all-white spacesuit, were also to re-route cables and lubricate key moving parts on the space station.

The spacewalk was scheduled to last six-and-a-half hours.

Choreographing the spacewalk and coordination with Mission Control in Houston was mission specialist Randy Bresnik, who woke the crew up earlier with his choice of song for the day, "In Wonder" by the Newsboys.

Meanwhile, back on the space station, crucial rewiring was being carried out on the hatch where the new Tranquility node is to be connected early next year, allowing more berths for astronauts and an all-round view of the station.

The crew of Atlantis docked on Wednesday at the International Space Station as part of an 11-day voyage to deliver a 20,000-pound (9,071-kilogram) haul of spare parts.

With only five launches left before the 2010 retirement of the shuttle fleet, NASA officials said the parts were essential for extending the life of the space station.

This fifth and final shuttle mission for 2009 is scheduled to include three space walks to store hardware on the exterior of the space station and bring US astronaut Nicole Stott, who has been on the ISS since August, back to Earth.

Stott, who celebrated her 47th birthday Thursday in space, and the rest of the crew will also be conducting scientific experiments that could explain muscle loss in space.

Thousands of the microscopic creatures have been sent from Britain's University of Nottingham to study the effect of zero gravity on the human body's muscle development and physiology.

The worms will be stashed inside the Japanese Kibo laboratory on the ISS where they will be tested with several potential treatments for muscle loss.

NASA's shuttle program is due to be mothballed next year, but the White House could still decide to extend it through 2011 to reduce America's future reliance on Russia for transporting astronauts to the space station.

Last month NASA successfully launched the prototype Ares I-X rocket that it hopes will transport a new generation of manned exploration vehicle called the Orion, which is still in development.

Orion, which will not be ready at least until 2015, is being designed to take a crew of up to six astronauts on flights to the International Space Station, or a crew of four on lunar missions lasting up to 210 days.

The human space flight program, which swallows up 10 billion dollars of NASA's 18-billion-dollar annual budget, is at great risk of being grounded.

A panel set up by President Barack Obama and tasked with assessing the future of US human space flight has said an additional three billion dollars a year is needed for NASA to meet its goals.

All rights reserved. © 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.